Process for plating tin and tin alloys



March 3, 1942. w HOPPER PROCESS FOR PLATING TIN AND TIN ALLOYS FiledAug. 10,.1958

, INVENTOR BY [Did 4R0 WfiOPfif/R ATTORN Ys Patented Mar. 3, 1942PATENT" OFFICE PROCESS FOR PLATING TIN AND TIN ALLOYS Edward WilliamHopper, Midland, Pa., assignor,

by mesne assignments, to Crucible Steel Company of America, acorporation of New Jersey Application August 10, 1938, Serial No.224,058

9 Claims.

This invention relates to method and apparatus for the production ofsmooth, bright, impervious and adherent tin or tin alloy coatings ofuniform thickness on iron or steel sheetor strip by a processinvolving,electrodeposition of the tin or tin containing coatingdirectly on the iron or steel base metal.

While the manufacture of tinplate by electrodeposition of the tinpossesses advantages over the earlier practice of coating by hot dippingthe base metal in a molten bath of tin, the electroplating process ashitherto practised has the disadvantage that the coating obtained is ofnonuniform thickness and is lacking in brilliance and in adhesion to theiron or steel base. It has.

been sought to overcome these defects by heating the electrically coatedarticle above the melting point of the coating, for example by immersionin a hot oil bath, of appropriate characteristics, such as palm oil,with the object of rendering the-coating bright and adherent by fusion.It has been found, however, that the molten metal contracts aboutnumerous focal points forming raised beads or drops of the coating metalthereat separated by thinly coated or bare areas, the latter exposingthe base metal. The surface thus assumes a grainy appearance like thatof an orange peel. This phenomenon is not confined to fused,electroplated coatings but applies as well to coatings produced by hotdipping. The explanation of this effect as well as the variation incoating thickness, is to be found in the nonuniform character of thebase metal. Although strip steel, such as is used for tinplate, is lowin carbon, phosphorus, sulfur, silicon, etc., nevertheless such elementsare present in sufficient amount as, in conjunction with the otherimpurities unavoidably present, such as slag inclusions, rolled-inscale, seams, pipes, etc., to render the surface thereof quitenon-uniform from point to point as regards chemical composition wherebyvariations in surface tension and consequent wetting action with respectto the molten coating metal are present causing the metal to contractinto beads as aforesaid, and whereby variations in surface potential areset up to render the thickness of the coating non-uniform.

In an attempt to overcome these defects various intermediate coatings ofothermetals, such as copper, nickel, etc. and alloys thereof, have beenapplied between the base metal and the final coating of tin or tinalloy. Also it has been sequently brightened by fusion. All of theseexpedients are, however, unnecessarily expensive, because they involveprocedures, equipment, labor and material which I have found to beunneccessary.

, With my invention coatings of tin and alloys thereof may beelectrically deposited directly on iron or steel and thereafterbrightened by fusion without necessity for deposition of intermediatecoatings of other metals or fusion of successive coatings, such as isrequired by practices heretofore known.

According to my invention the base metal strip, suchas the low carbonstrip steel'used for tinplate, is especially prepared and cleaned forplating by subjection to a pretreatment consisting in the successivesteps of electrolytic cleaning in an alkaline bath, scrubbing in water,acid pickling, scrubbing in water, annealing or normalizing followed bya slight pinch roll on cooling, electrolytic recleaning in an alkalinecleaner, scrubbing in water, acid pickling and. a final scrubbing inwater. This procedure has for its object the imparting of a cleansurface to the strip containing as little foreign matter and inclusionsas possible. The steps above recited preceding the annealing operationmay be omitted, but the results are not so satisfactory as when thesesteps are included.

The strip as above prepared is now passed into an alkaline tin platingbath and plated uniformly over its entire surface. Inasmuch as analkaline bath has better throwing power than the more commonly employedacid baths, a more uniform coating of finer particle size is thusobtained, including the surface areas aforesaidoffering greaterresistance to electrolytic deposition. I have found in fact, that steelstrip when cleaned and otherwise prepared in the manner above outlined,may be coated to commercial thicknesses in an alkaline bath, andthereafter brightened by fusion without formation of the grainy surfaceproposed to first fuse an extremely thinly deposited coating of tinalloy to the base metal, followed by deposition of a heavier coatingsubor orange peel" effect above referred to. If, for example, the stripso coated is passed through a bath of Hot oil, such as palm oil,maintained at a sufficiently high temperature to fuse the coating, andthen through a bath of cool oil, that the hardened coating so obtainedwill be smooth, adherent and of high lustre approaching mirrorbrightness.

Inasmuch, however, as alkaline tin plating baths are difficult tomaintain and are of relatively high electrical resistance andcorrespondingly low plating efllciency and deposition rate as comparedto acid baths, I prefer to employ the alkaline bath only to apply aflash coating to the strip metal, whereupon the strip after being rinsedand possibly scrubbed, is passed into an acid plating bath for platingto the requisite thickness. In the drawing, Figs. 1, 2 and 3 showdiagrammatically, progressive portions of the apparatus through whichthe strip to be plated is fed during the cleaning, pickling, annealing,recleaning and pickling, plating and brightening operations. Fig. 1shows apparatus for the initial cleaning, pickling and annealing; Fig. 2that for the recleaning, pickling and plating; and Fig. 3 the apparatusfor brightening and cleaning the plated strip.

Referring now to the figures of the drawing, which show diagrammaticallya preferred form of apparatus for practicing the invention, strip steelI, such as is used for tin plating, is fed from a roll thereof 2, overguide rolls, 3 to I inclusive, into an electrolytic alkaline cleaningbath 3, thence within the bath between a pair of metal plate electrodes9, III, which may be of stainless steel or the like, and out of the bathover rolls ll, l2. The electrodes 3, II) are connected to the positiveterminal of a direct current generator l3, the negative terminal ofwhich is connected to the guide roll I, thereby to complete anelectrical circuit through the bath between the steel strip l and theelectrodes 9, III, whereby the strip is subjected to cathodic cleaning.I prefer to use for bath 8 a solution of sodium ortho-silicate,containing a small amount of a cresylic acid compound, the latter forimproving the emulsifying properties of the bath. I may employ forexample, 8 ounces of the silicate per gallon of water in the bath, andadd thereto 1% by weight of the cresylic acid compound. I have foundsuch compounds sold under the trade names of Dri-Sil and'Quaker 700-)!to be satisfactory.

After leaving the cleaning bath 8 the steel strip i is preferably fedthrough a scrubber [4 wherein it is scrubbed by .wire brushes l5. I3,between which and the cooperating rolls l1, la the strip is passed, in awater spray supplied from nozzles I9, 20.

On leaving the scrubber the strip is fed by rolls 2| to 24 inclusivethrough a pickling bath 25, in order to remove all possible rolled-inscale, rust, cake particles and other surface impurities. I preferablyuse in the bath 25, a hydrochloric acid or a sulfuric acid plus sodiumnitrate pickle. Following this operation the strip is fed through ascrubber 26 similar to II, to remove the pickling solution and any looseand insoluble surface material. The strip is then rinsed by feeding overrolls-21 to 30 inclusive, through a water bath 3|, and is then dried bypassage through a dryer 32 heated by steam pipes 33.

After drying the strip is preferably annealed or normalized in areducing atmosphere by passage through a furnace 34, in order to relievethe eifects of cold rolling and to impart softness, deep drawing andforming properties thereto. In the event it is found desirable to applythis treatme'nt as a separate, isolated operation, the strip may bereeled up after passage through the dryer 32 and subsequently unreeledfor passage through the furnace as indicated by the break in the stripat 35. Likewise the strip may be rereeled after passage through thefurnace and th'ereafter unreeled for subsequent treatment as indicatedby the break in the strip at 36.

After the strip has cooled following the far- 75 nace treatment, it ispinch rolled, i. e. rolled slightly thinner, by passage between polishedrolls 31, to impart a fine, fiat surface finish and to improve thephysical properties of the strip.

It is then cathodically recleaned by passage through an alkaline bath38, identical in construction and arrangement with bath 8 above referredto. The strip is then scrubbed and washed in water by passage through a.scrubber 39 similar to l4, and is then fed through an acid pickling bath40. In this bath I prefer to use a 17% hydrochloric acid solutionmaintained at about 190 F., the bath being of such dimensions inrelation to the speed of the feed of the strip, that the strip isimmersed therein for about 5 110.10 seconds. The pickled strip is thenscrubbed and washed by passage through a scrubber 38 similar .striptreated by this method and followed by a brightening process inaccordance with my invention, gives a better wetting or alloying action,resulting in more even coating and a smooth, mirror finish whenbrightened, in contrast to the orange peel and porous coatings generallysecured.

The strip steel as thus treated is now passed into an alkaline tinplating bath 42, and preferably irnmersed th'erein only for a suflicientinterval to impart a flash coating such, however, as to insure completecoverage and uniformity of coating over the entire steel surface. I haveused the following bath, plating for 5 to 20 seconds at amperes per sq.ft. and at a temperature of about 160-180 F.:

Na-rSnOa oz./gal 33.5

NaOH.. do 2.80 NazCOa ..d0.. 0 to 10.0

The plating is effected in the usual manner by passing the strip withinthe bath between metal electrodes 43, 44, such as copper, having sheets45 of the coating composition, 1. e., tin or alloy thereof aflixedthereto, these electrodes being connected to the positive terminal of adirect current generator 46, the negative terminal of which is connectedto one or more of the feed rolls 4! over which the strip is fed whileimmersed in the bath.

After the flash coating has thus been imparted,

issimilar to that of the alkaline bath 42. For

the acid bath I prefer to use the following:

SnSO4 lr oz. /gal 20.0 H2804 ..--d0- 3 Cresylic acid do 0.3.HzSO4-i-C16SY11C acid approx 0.8N

Hydroquinone oz./gal 0.1 Sulfonated alcohol by volume 3 The plating ofthis bath is carried out for about 20 to 60 seconds at about 30 toamps/sq. ft., depending on the speed of the strip, coating thicknessdesired, etc., and at a temperature of about 70 to 100 F.

The thus coated strip is now scrubbed and washed by passage through ascrubber 50 similar to H, and therafter rinsed in a water bath followingwhich it is dried by passage through a steam-heated dryer 52 similar to32.

The strip is now ready for the brightening process which is effected bypassing the strip over a feed roll 53, and thence in a straight linethrough a suitable hot oil bath 54, for example palm oil, and through aduct 55 interconnecting the hot oil bath with a cool oil bath 56 to afeed roll 51 immersed in a cool oil bath, under which the strip ispassed and fed out of this bath between a pair of squeegee rolls 58 forremoving the excess oil adhering to the strip.

The hot oil bath 54, is maintained at a temper ature suiilciently high,for example, 475 to 500 F., to fuse the tin coating to a smooth, brightadherent surface; while the cool oil bath 56 is maintained'at atemperature sufiiciently low, for example 135'to 150 F., to quicklyharden and solidify the coating, thereby permanently to retain thefinish imparted in the hot oil bath. I have I 'found palm oil to besuitable for these baths.

The strip of course must not be permitted to pass over feed rolls orinto contact with other hard objects whilethe coating is in the plasticor molten condition resulting from immersion in the hot oil bath,because this would produce imprints of the rolls, etc. in the coatingand thus mar the finish. Likewise the coating must be cooled rapidly ina non-oxidizing atmosphere to retain its brilliance and freedom fromoxidation and crystallization or spangling of the surface such as wouldotherwise occur, This necessitates maintaining the hot and cool oilbaths in liquid contact so that the strip may be passed directly fromone to the other without passage over rolls or being exposed to theatmosphere. This is ac-.

complished-by means of the duct 55 which extends downwardly from the hotoil to the cool oil bath to such adegree that its entry 59 into the cooloil bath is situated entirely below its entry .60 into the hot oil bath.As is more fully dethe strip then passing out of bath 63 and between apair of squeegee rolls 1 l, for removing the excess solvent, and betweensteam-heated plates 12 of an evaporator 13 for drying the strip isreeled'up as indicated at 14 after passage over guide rolls 15.

The baths are so arranged as to provide a flow of the cleaning liquidbetween them in the direction opposite to that in which the strip isfed. This is accomplished by feeding purified cleaner, preferablycontinuously, into bath 63 from a supply line 16, a correspondingportion of the liquid in bath 63 in'turn flowing into bath 62 throughand-around the felt members 59. A like portion of the liquid in bath 62in turn flows into bath 6| over the low barrier 11 separating thesebaths, while a corresponding portion of the liquid in bath 6| flows intobath 60 through and around the felt members 65. Bath 60 in turn isprovided with an outlet pipe 18 by means of which a correspondingportion of the liquid in this bath is drawn oif,- preferablycontinuously. As a result of this arrangement, the strip is given anexceptionally efiicient cleaning in passing through the baths, inasmuchas the'most contaminated cleaning' liquid accumulates in bath 60 whichthe uncleaned strip first enters, whereby the principal portion of thegrease, etc., thereon is removed, the strip thereafter passing intoprogressively cleaner baths as the stripitself becomes progressivelycleaner. The novel features of these baths and the associated evaporator13 are described and claimed in copending application Serial No.

scribed Patents 2,141,382 and 2,192,303, an inter- I connecting duct soarranged, provides an effective oil seal between .the hot and cool oilbaths 224,100, filed August 10, 1938.

What I claim is: v

1. The method of producing smooth, bright adherent coatings of tin andalloys predominantly of tin, on iron and steel strip, which comprises:electrolytically depositing a thin coating of the coating material onsaid strip from an alkaline plating bath, superimposing thereon athicker coatingof said material electrolytically deposited from an acidplating bath, immersing the so-coated strip in a hot bath of fatty acidcontaining, non-oxidizing oil to fuse and brighten said coating, andthereafter immersing said strip in a bath of cool non-oxidizing oil toharden said 4 coating in said brightened condition.

such as substantially to prevent thermal .ex-'-" from nozzles 55a at itsentrance into the coql oil=-"" baths, and drawing oil oil at anintermediate point between the baths, asby means of the outlet pipe 55b.

The-strip on emerging from the squeegee rolls adhering thereto and must,therefore, be given a careful cleaning before it is ready for use. This'is accomplished by passing the strip through a succession of baths 60to 63 inclusive, containing a liquid solvent, such as carbontetrachloride. The strip enters bath 60, passing under a feed roll 64 2.The method of producing smooth, bright,

"adherent coatings of tin and alloys predominantly of tin, on ironand.steel strip, which comprises: electrolytically, depositing a thincoating of the coating material on said strip from an alkaline platingbath, superimposing thereon a thicker coating of said materialelectrolytically deposited from an acid plating bath, and immersing theso-coated strip in a hot bath of a fatty acid containing, non-oxidizingoil to fuse and brighten said coating.

3. The continuous method .of producing smooth, bright, adherent coatingsof tin and al- 58, still has considerable oil and other impurities loyspredominantly of tin, on iron and steel strip,

which comprises: progressively feeding said strip and between a pair offelt or like members 65 dis- "posed in an orifice of a partition 66separating baths B0 and 6|, the strip thus passing by means of the feedrolls 68 out of bath BI and into bath 62, thence from bath 62 into bath63 between a pair of felt or like members 59 disposed in an orifice in apartition I0 separating baths 62 and 63,

through an alkaline plating bath to deposit a thin coating of thecoating material thereon, feeding the strip thence through an acidplating bath to superimpose thereon a thicker coating of the coatingmaterial, and feeding the strip thence through a hot bath of fatty acidcontaining nonoxidizing oil to fuse and brighten said coating.

4. The continuous method of producing smooth, bright, adherent coatingsof tin and alloys predominantly of tin, on iron and steel strip, whichcomprises: progressively feeding said strip through an alkaline platingbath to deposit a thin coating of the coating material thereon, feedingthe strip thence through an acid plating bath to superimpose thereon athicker coating of the coating material, feeding the strip thencethrough a hot bath of fatty acid containing, nonoxidizing oil, to fuseand brighten the coating, and feeding the strip thence, undernon-oxidizing conditions and free from mechanical contact, into a bathof a cool, non-oxidizing oil to harden said'coating in said brightenedcondition.

5. The. continuous method of producing smooth, bright, adherent coatingsof tin and alloys predominantly of tin, on iron and steel strip, whichcomprises: progressively feeding a single thicknessof said strip througha normalizing furnace to anneal the same, thence cleaning the stripelectrolytically in alkaline solution followed by acid pickling, feedingthe strip thence through an alkaline plating bath to deposit a thincoating of the coating material thereon, feeding the strip thencethrough an acid plating bath to superimpose thereon a thicker coating ofthe coating material, feeding the strip thence through a hot bath offatty acid containing non-oxidizing oil to fuse and brighten thecoating, and feeding the strip thence, under non-oxidizing conditionsand free from mechanical contact, into a nonoxidizing bath of a cooloil, thereby to harden said coating in said brightened condition.

6. The method of producing smooth, bright, adherent coatings of tin andalloys predominantly of tin,'on iron and steel strip, which comprises:annealing the strip and electrolytically cleaning inlalkaline solution,acid pickling the strip, clectrolytlcally depositing thereon a thincoating of the coating material from an alkaline platlngbath,superimposing thereon a thicker coating of said materialelectrolytically deposited from an acid plating bath, immersing thesocoated strip in a hot bath of fatty acid containing non-oxidizing oilto fuse and brighten said coating and thereafter immersing said strip ina bath of cool, non-oxidizing oil to harden saidv immersing said stripin a cool bath of non-oxielectroplated from an acid plating bathdirectly onto the coating plated from said alkaline plating bath.

9. The method of producing smooth, adherent coatings of tin and alloyspredominantly of tin,

on a ferrous base metal, which comprises: electrolytically depositing athin coating of the coating material on said ferrous base metal from analkaline plating bath, and superimposing thereon an overcoating of saidcoating material electrolytically deposited from an acid plating bath.

EDWARD WILLIAM HOPPER.

- Patent No; 23714365- CERTIFICATE-OF CORRECTION.

March 5, 1914.2.

EDJARD WILLIAM HOPPER..

"It is hereby certified that error appears in the prin sp cific' tion ofthe above. numberedpatent requiring correction as follows: Page 3, sec0nd column, line 1;, e fterfstrip'f insert following which the strip;

' and that the acid. Letters Patent shouldbe read with this correctiontherein that tbe same may conform to the record of the case in thePatent Office.

Signed and sealed this 21st day of April, A. n. 19h2.

. Henry Von Airedale,-

xs I Actingcommissioner of Patents.

